The present invention relates to a method of making tubular products, especially suited for the manufacture of catheters. More particularly, this invention relates to a method for making a reinforced tubular product, such as a reinforced catheter, in a single extrusion step wherein the location of the reinforcement in the wall of the tubular product is simply but accurately controlled. Still more particularly, this invention relates to a method for making a catheter of the type wherein the reinforcement is omitted from the distal end of the catheter through intermittent application of the reinforcement during the manufacturing process. Still more particularly, this invention relates to a method for making a reinforced tubular structure, such as a catheter, wherein the reinforcing material is applied by helically wrapping an intermediate structure with at least a length of reinforcing material and controlling the angle of helical wrapping to control selected physical characteristics of the finished product.
The art of manufacturing tubes, pipes or cannulae by extruding a plastic material to produce significant quantities of the tubing is fairly well developed. In many instances, it is desirable to use reinforcement in the tubes or pipes to increase the pressure, tensile, or torque carrying capacities of those tubes or pipes. Ordinary plastic garden hose reinforced with filament is a common example of such a product made according to prior art techniques.
A special type of tubular product, as is also well known in the art, is a catheter for application in the medical field. Catheters of the type contemplated are relatively thin and flexible tubes which include inner and outer plastic layers with a wire sheathing embedded between the layers. The wire sheathing is either braided or cross-wound to obtain a maximum torsional rigidity and a satisfactory longitudinal flexibility. However, such catheters and in particular those intended for diagnostic purposes, which use radio-opaque dyes and fluoroscopy, to deliver the radio-opaque dyes through the reinforced extruded catheter to the desired site have particular constraints on their construction. Since the dyes are relatively viscous, and the speed of injection is important, high pressures are developed in the relatively small tubing during delivery of the dye. To resist these high pressures, but to retain a degree of flexibility which permits the catheter to be used within a body, it is necessary to reinforce the flexible materials used in the catheter rather than use materials strong enough in their reinforced state. The latter materials are usually too stiff for use in the vascular system of the human body. Thus, in general, high strength tubes are too stiff for medical use, while highly flexible soft polymers have insufficient strength characteristics so that reinforcement of the softer materials with a higher strength reinforcing material is necessary.
All of the foregoing constraints are measured against the desirability in the art to make the outside diameter of the catheter, including its tip, as small as possible to reach very narrow passages in the body. In that regard, the flexible catheters produced according to the art, in addition to exhibiting satisfactory pressure resistance, must also be able to transmit axial torque from the end remaining outside of the body, i.e. the proximal end, to the other end of the catheter, i.e. the distal end, located inside the body. This torque transmission is required to allow the delivery tip of the catheter to be guided through the twists, turns, valves, and bends in the vascular system of a patient to arrive at the point where dye injection is desired. Such catheters generally have braided metal reinforcement in the wall of the small diameter catheters to withstand pressures of 1000 psi or more.
A typical prior art process for making a reinforced extruded catheter is a three step process. In the first step, a mandrel having an outside diameter equal to the desired inside diameter of the finished catheter is passed through suitable extrusion tooling to cause a tubular jacket or sheath of the catheter material to form around the mandrel. In this step, the outside diameter of the first extrusion layer on the mandrel is smaller than the desired finished outside diameter of the finished catheter. As an alternative, a non-mandrelized tube can be extruded with an ID and OD which are the same as that of the mandrelized tube. Next, the inner core tube formed in the first step as described above is processed by suitable machinery to cause a pattern of reinforcing material, such as wires, fibers, or monofilaments, for example, to be laid along and/or around and in contact with the surface of the core tube. Next, the composite intermediate structure of the inner core tube and the reinforcing layer thus applied is again passed through suitable extrusion tooling equipment to deposit a second layer of catheter material around and bonded to the composite thereby encapsulating the now-reinforced inner core tube forming essentially a single structure. The outside diameter of the second layer of extrusion is approximately equal to the desired finished outside diameter of the catheter. Subsequently, finishing and polishing operations can be performed and the composite thus constructed cut to its desired length. The mandrel, if any, is then extracted by lengthwise pulling, leaving the hollow catheter tubing with reinforced walls.
Most catheters require a flexible non-reinforced tubular tip on the end used in the body. Thus, a welding operation is then performed whereby non-reinforced tubing of similar overall dimensions to that of the reinforced body is added to the composite structure, such as by plastic welding. This provides a flexible tip for steering the catheter through the cavities in the body of the patient. It has, however, been a problem in the art that such weldments between the tip and the catheter body are sometimes weak causing the tip to break off during the catheterization procedure.
An advance in the method of manufacture is represented in U.S. Pat. No. 4,321,226 to Markling with regard to the manufacture of a tip wherein wire sheathing is omitted in the tip or distal end. The method there described provides for removing a portion of the wire braid through predetermined lengths of the catheter, such as by cutting, to leave defined, spaced, areas of the catheter without wire reinforcement for subsequent encapsulation by another extrusion. Thus, a unitary catheter, with an end portion without wire reinforcement adjacent to a portion with such wire reinforcement, is produced.
The art of applying braiding or multi-pass wire reinforcement to a catheter inner core is also well developed and machinery for performing such a step is well known. In general, such reinforcement material is applied to the inner core tube in a pattern of right and left hand helices, each having one or more reinforcements applied over a given area or length of the tube or cannula. However, a typical braiding process causes all reinforcements to be laid down in a direction which passes alternatively over and under all of the reinforcements laid down on the inner core tube in an opposite direction. Thus, the machinery performing the braiding process must move the reinforcements alternately radially inwardly and outwardly, as well as circularly, whereby the tension on the reinforcement continuously varies at a given production speed, and further increases as production speed increases. Such variations, in the extreme, cause the reinforcement to break, especially as the production speed increases. Moreover, the braiding machinery is subjected to significant internal inertial loads, shock loads, and lubrication requirements, especially because of the requirement to generate two directions of movement of the reinforcements at the same time. These shock and inertialloads put a definite upper limit on the speed of the machinery and therefore limit its production rate.
Still another shortcoming of braid-reinforced catheters arises from the oppositely-oriented application of the reinforcing braid resulting in difficulties in transmitting axial torque along the catheter while in use. When such a braided catheter is twisted, one of the helices of the wrap will be tensioned while the other is compressed, resulting in a loss of angular control through the catheter in steering the catheter through the human body, due to inefficiencies in transmitting torque through the elongated catheter body.
Still another difficulty with prior art catheters results from the inherent compromise between stiffness and torque transmission due to the helix angle of the braid. As the helix angle (i.e. the angle of the reinforcement relative to the axis of the catheter) decreases, the apparent stiffness of the catheter increases, but the efficiency of torque transmission decreases. Thus, a tube or catheter with wire parallel to its axis yields the maximum stiffness but the minimum torque transmission efficiency; conversely, a circular wrap has not only the highest torque transmission efficiency but also the poorest stiffness characteristics. Thus, the angle of helical braid generally selected in conjunction with the polymer used is a compromise between these two factors and thus limits the polymers which may be used. It is an aim of the invention to utilize a wrapping technique for applying reinforcement at a high angle (i.e. nearly circular wrap) while also applying a second wrap at a lower angle (more axial) to make integral tip catheters with soft polymers previously unsuitable for use in catheters.
Thus, it is an overall objective of this invention to provide a reinforced catheter of the type described in which the reinforcement materials are provided on an intermittent basis along the body of the catheter and wherein the radial location of the reinforcement layer in the wall of the catheter is precisely controlled, while at the same time reducing the number of steps in the production process.
It is another objective of this invention to provide a method for making a catheter by heating a catheter tube and a reinforcing member while tensioning the member as the reinforcing member is applied to the tube, so that the member deforms or penetrates the softened tube wall to a depth controlled by the degree of tensioning and the plasticity of the tube, whereby, after smoothing and sizing, a single extrusion of a tube at essentially the finished size can produce a reinforced composite catheter.
It is another overall objective of this invention to produce a tube by the process described wherein the pattern of the reinforcement is interrupted to provide a straight line of reinforcement at the surface of the tube to permit easy removal of such straight portion when producing, for example, an integral tip catheter.
It is still another objective of this invention to apply a helical wrap to a body of tubular material at a large angle approaching a perpendicular to the axis of the tube and applying a second helical wrap at a small angle approaching the axis of the tube, whereby torque and stiffness of the tube are maximized while expanding the class of polymers which may be used.
These and other objectives of this invention will become apparent from the drawings and the detailed description of the invention.